Chapter 6

Why Cells Divide:

  • growth and repair
  • creation of gametes (sex cells)
  • method of reproduction in unicellular organisms

Binary Fission - type of reproduction that occurs in bacterial cells, single celled organism splits and becomes two identical organisms

Chromosomes and DNA

Chromosomes are DNA wrapped around proteins to form an X-shaped structure.


The diagram will help you see the relationship.

1. Chromosomes are found in the nucleus
2. Chromosomes are made of DNA
3. Sections of chromosomes are called genes


DNA - deoxyribonucleic acid (it is the genetic code that contains all the information needed to build and maintain an organism)



Chromosome Structure



Chromosome Numbers

Each organism has a distinct number of chromosomes, in humans, every cell contains 46 chromosomes. Other organisms have different numbers, for instance, a dog has 78 chromosomes per cell.

Somatic Cells - body cells, such as muscle, skin, blood ...etc. These cells contain a complete set of chromosomes (46 in humans) and are called DIPLOID.

Sex Cells - also known as gametes. These cells contain half the number of chromosomes as body cells and are called HAPLOID

Chromosomes come in pairs, called Homologous Pairs (or homologs). Imagine homologs as a matching set, but they are not exacly alike, like a pair of shoes.

Diploid cells have 23 homologous pairs = total of 46

Haploid cells have 23 chromosomes (that are not paired) = total of 23

Homologous Chromosomes


Sex Determination

Chromosomes determine the sex of an offspring. In humans, a pair of chromosomes called SEX CHROMOSOMES determine the sex.

If you have XX sex chromosomes - you are female

If you have XY sex chromosomes - you are male

During fertilization, sperm cells will either contain an X or a Y chromosome (in addition to 22 other chromosomes - total of 23). If a sperm containing an X chromosome fertilizes an egg, the offspring will be female. If a sperm cell containing a Y chromosome fertilizes an egg, the offspring will be male.

Creation of a Zygote

When two sex cells, or gametes come together, the resulting fertilized egg is called a ZYGOTE

Zygotes are diploid and have the total 46 chromosomes (in humans)


A karyotype is a picture of a person's (or fetus) chromosomes. A karyotype is often done to determine if the offspring has the correct number of chromosomes. An incorrect number of chromosomes indicates that the child will have a condition, like Down Syndrome

Compare the Karyotypes below




Notice that a person with Down Syndrome has an extra chromosome #21. Instead of a pair, this person has 3 chromosomes - a condition called TRISOMY (tri = three)

Trisomy results when chromosomes fail to separate - NONDISJUNCTION - when sex cells are created. The resulting egg or sperm has 24 instead of the normal 23.

Other conditions result from having the wrong number of chromosomes:

Klinefelters Syndrome - XXY (sex chromosomes)

Edward Syndrome - Trisomy of chromosome #13


**Try constructing your own Karyotype at


Go to the Cell Cycle